1. Field of the Invention
The present invention relates to a light modulator, more particularly a waveguide-type light modulator which can produce a plurality of independently modulated light beams, and a recording device which incorporates such a light modulator.
2. Description of the Prior Art
One conventional light modulator which can divide a single light beam into a plurality of light beams and independently modulate the divided light beams, incorporates a bulk-crystal-type acoustooptic device. In the known light modulator, ultrasonic waves with combined frequencies are propagated in the bulk-crystal type acoustooptic device, and a light beam introduced into the device is Bragg-diffracted in different directions by the ultrasonic waves. When the ultrasonic waves having different frequencies are turned on and off, i.e., intermittently propagated, the respective diffracted light beams are turned on and off or modulated.
Japanese Unexamined Patent Publication No. 63(1988)-103208 discloses a light modulator employing an optical waveguide for modulating a plurality of light beams. The disclosed light modulator serve as a portion of an optical write head. The light modulator comprises an optical waveguide made of a material capable of propagating a surface elastic wave, a surface elastic wave generating means responsive to an applied high-frequency voltage for generating, in the optical waveguide, a surface elastic wave having a frequency corresponding to the frequency of the applied high-frequency voltage, and for directing the generated surface elastic wave across the light path of a parallel beam of light which is guided in the optical waveguide, a driver circuit for applying a plurality of high-frequency voltages having different frequencies to the surface elastic wave generating means, and a modulating means for turning on and off or modulating the high-frequency voltages.
In the light modulator which employs the bulk-crystal-type acoustooptic device, a transducer which generates ultrasonic waves is required to be bonded to an acoustooptic medium, and then to be ground to a predetermined thickness highly accurately. If a wide frequency band is desired, then the thickness of the transducer ranges from several microns to several tens of microns. However, grinding the transducer to such a very thin layer with high accuracy requires a very sophisticated grinding technique. The light modulator of this type cannot be manufactured at a high production rate and is very expensive.
The light modulator which employs the optical waveguide disclosed in the above publication is free of such a problem because it employs an interdigital transducer (IDI) as the surface elastic wave generating means. More specifically, the IDT can easily be fabricated by electron beam patterning or photolithography since these fabrication processes have already been established and widely used for the fabrication of semiconductors.
In the light modulator disclosed in the above publication, all generated surface elastic waves having different frequencies are propagated in one direction. Therefore, the light beam guided in the optical waveguide may be diffracted very efficiently by a surface elastic wave having a certain frequency, but may not be diffracted very efficiently by a surface elastic wave having a widely different frequency. Such a drawback is eliminated when the band of combined surface elastic wave frequencies is reduced. If the speed at which the light beam is modulated is constant under such a condition, however, the number of light beams which ca be divided from the introduced light beam is reduced, and the light modulator has a limited performance. The maximum number N of divided light beams is expressed by: EQU N=.tau..multidot..DELTA.f+1 . . . (1)
where .DELTA.f is the band of surface elastic wave frequencies, and .tau. is the time required for the surface elastic wave to travel across the light beam (.tau.=D/v: D is the width of the light beam, and v is the speed of the surface elastic wave). According to the equation (1), the more the divided light beams (i.e., the greater the band .DELTA.f) and the higher the speed of modulation (i.e., the shorter the time .tau. since the modulation speed .perspectiveto.1/.tau.), the higher the performance of the light modulator becomes.
In the case where the light modulator is incorporated in a recording device, for example, because more dots can be recorded in a given period of time as there are more divided light beams and the speed of modulation is higher, if the band .DELTA.f of surface elastic wave frequencies is small then the number of recorded dots is reduced, and the practical value of the light modulator is lowered.
With the arrangement disclosed in the above publication, the surface elastic waves having different frequencies ar generated from different electrode fingers of the IDT, and the time required for the surface elastic waves excited by the IDT to reach a location where they diffract the light beam, varies from surface elastic wave to surface elastic wave. Even if the high-frequency voltages of different frequencies are applied to the IDT at the same time, therefore, the guided light beam is diffracted at different times by the surface elastic waves. Consequently, the divided light beams are modulated at different times. If an image is recorded using such a plurality of modulated light beams, the recorded image will be distorted.